1. Field of the Invention
The present invention relates to a multiphoton-absorption exposure method and a multiphoton-absorption exposure apparatus for exposing a recording medium or the like by using multiphoton absorption (such as two-photon absorption), which is known as one of the nonlinear optical effects.
2. Description of the Related Art
The following documents (1) to (4) disclose information related to the present invention.    (1) U.S. Patent Laid-Open No. 20010001607    (2) U.S. Patent Laid-Open No. 20030052311    (3) “Three-dimensional Microfabrication with Two-photon-absorbed photopolymerization” by S. Maruo et al., (OPTICS LETTERS, Vol. 22, No. 2, pp.132–134, Jan. 15, 1997)    (4) “Two-photon-absorption Optical-fabrication with a Micro-lens Array” by Y. Adachi et al., (Extended Abstracts of the 50th Spring Meeting of the Japan Society of Applied Physics and Related Societies, 27p-YN-4, March 2003)
Although only one photon is absorbed in most light absorption processes occurring in materials, exposure to high-power light such as ultrashort pulse laser light can cause the so-called multiphoton absorption, in which two or more photons are simultaneously absorbed. For example, the amount of energy which a material receives in the two-photon absorption is twice the amount of energy which a material receives in the single-photon absorption, and reactions of two-photon absorption caused by exposure to high-power light having a wavelength 21 (in which the photon energy is half of that in the light having a wavelength 1) to a material which primarily absorbs the light having the wavelength 1 are equivalent to reactions caused by exposure to the light having the wavelength 1.
The probability of multiphoton absorption increases in proportion to the photon density. Therefore, when converging light is applied to a multiphoton absorption material, it is possible to selectively cause multiphoton absorption only in the vicinity of a convergence position of the converging light, where the photon density is maximized at the convergence position. Thus, when a recording material is realized by a material in which a change in the phase, the refractive index, the chemical state, or the like is caused by multiphoton absorption, it is possible to record information in a plurality of layers in the recording material by exposing the recording material to converging light, and specifically by scanning each layer with the converging light and shifting the depth of the convergence position to the next layer in order to scan the next layer. The aforementioned document (1) discloses an example of an apparatus which records information by scanning a multiphoton absorption material with converging light for exposure as mentioned above.
In other disclosed methods, three-dimensional optical molding of a multiphoton absorption material in which a photopolymerization reaction occurs is performed by three-dimensionally exposing the multiphoton absorption material to converging light, where the above-mentioned fact that multiphoton absorption is caused only in the vicinity of the convergence position of the converging light is utilized. The aforementioned documents (3) and (4) disclose examples of apparatuses for performing three-dimensional optical molding of a multiphoton absorption material as mentioned above. In addition, the document (4) also discloses a method for splitting a single laser beam into a plurality of laser beams by use of a microlens array, and concurrently molding a plurality of three-dimensional structures by use of the plurality of laser beams.
Further, the document (2) discloses examples of preferable multiphoton absorption materials which have great two-photon absorption cross sections. For example, when light-curing resin is mixed into the multiphoton absorption materials, such multiphoton absorption materials become applicable to optical molding.
However, conventionally, when three-dimensional, information recording, optical molding, and the like are performed by exposing multiphoton absorption materials as mentioned above, the probability of occurrence of optical reactions varies with the depth of the exposure in the multiphoton absorption materials. Therefore, the exposure becomes insufficient or excessive.